Can You Catch Syntax Errors Before `eval()`?

Catching Syntax Errors from eval()

Consider the following code snippets:

<code class="python">try:
    a+a=a
except SyntaxError:
    print("first exception caught")</code>

<code class="python">try:
    eval("a+a=a")
except SyntaxError:
    print("second exception caught")</code>

In the second case, the "second exception ... " statement is printed (exception caught), while in the first one it is not. Are the two syntax errors raised ("SyntaxError1" and "SyntaxError2") different? Is there a way to catch "SyntaxError1" (thus suppressing compilation-time errors)?

Answer

The difference between "SyntaxError1" and "SyntaxError2" lies in the timing of their occurrence. "SyntaxError1" is raised by the compiler before the try/except block is even created, while "SyntaxError2" is raised when the compiler runs as part of eval(), after the try/except block has been set up.

To catch syntax errors, including "SyntaxError1," the compiler must run twice. eval() is one method to achieve this. Other options include:

• Using the compile() built-in function.
• Importing the code as a module after writing it to a separate file.
• Using exec() or execfile().

By forcing the compiler to run twice, it becomes possible to catch syntax errors and handle them through try/except blocks.

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